External Charger for an Implantable Medical Device Having a Thermal Diffuser

ABSTRACT

An external charging system for an Implantable Medical Device (IMD) is disclosed having a thermal diffuser proximate to the primary charging coil for distributing heat from the primary charging coil. In an example, the primary charging coil is mounted to a first side of a circuit board, and the thermal diffuser is also connected to the first side and in contact with the primary charging coil. In one example, the thermal diffuser is a plastic material, such as an acrylic pad, with a high thermal conductivity and a low electrical conductivity. The thermal diffuser may also contact temperature sensors mounted to the first side of the circuit board.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 15/938,325,filed Mar. 28, 2018 (allowed), which is a non-provisional applicationbased on U.S. Provisional Patent Application Ser. No. 62/514,304, filedJun. 2, 2017. These applications are incorporated herein by reference,and priority is claimed to them.

FIELD OF THE INVENTION

The present invention relates to wireless external chargers for use inimplantable medical device systems.

INTRODUCTION

Implantable stimulation devices are devices that generate and deliverelectrical stimuli to body nerves and tissues for the therapy of variousbiological disorders, such as pacemakers to treat cardiac arrhythmia,defibrillators to treat cardiac fibrillation, cochlear stimulators totreat deafness, retinal stimulators to treat blindness, musclestimulators to produce coordinated limb movement, spinal cordstimulators to treat chronic pain, cortical and deep brain stimulatorsto treat motor and psychological disorders, and other neural stimulatorsto treat urinary incontinence, sleep apnea, shoulder subluxation, etc.The description that follows will generally focus on the use of theinvention within a Spinal Cord Stimulation (SCS) system, such as thatdisclosed in U.S. Pat. No. 6,516,227. However, the present invention mayfind applicability in any implantable medical device system, including aDeep Brain Stimulation (DBS) system.

As shown in FIGS. 1A-1C, a SCS system typically includes an ImplantablePulse Generator (IPG) 10 (Implantable Medical Device (IMD) 10 moregenerally), which includes a biocompatible device case 12 formed of aconductive material such as titanium for example. The case 12 typicallyholds the circuitry and battery 14 (FIG. 1C) necessary for the IMD 10 tofunction, although IMDs can also be powered via external RF energy andwithout a battery. The IMD 10 is coupled to electrodes 16 via one ormore electrode leads 18, such that the electrodes 16 form an electrodearray 20. The electrodes 16 are carried on a flexible body 22, whichalso houses the individual signal wires 24 coupled to each electrode. Inthe illustrated embodiment, there are eight electrodes (Ex) on each lead18, although the number of leads and electrodes is application specificand therefore can vary. The leads 18 couple to the IMD 10 using leadconnectors 26, which are fixed in a non-conductive header material 28,which can comprise an epoxy for example.

As shown in the cross-section of FIG. 1C, the IMD 10 typically includesa printed circuit board (PCB) 30, along with various electroniccomponents 32 mounted to the PCB 30, some of which are discussedsubsequently. Two coils (more generally, antennas) are show in the IMD10: a telemetry coil 34 used to transmit/receive data to/from anexternal controller (not shown); and a charging coil 36 for charging orrecharging the IMD's battery 14 using an external charger.

FIG. 2 shows an external charger 50 for the IMD 10 and shows theexternal charger 50 and IMD 10 in cross section with the externalcharger 50 wirelessly conveying power via a magnetic field 45 to the IMD10, which power can be used to operate the IMD and/or recharge the IMD'sbattery 14. The magnetic field 45 is generated by a primary chargingcoil 52. The external charger 50 contains one or more PCB 54 on whichelectronic components 56 are placed. See U.S. Pat. No. 9,002,445. Someof these electronic components 56 are discussed subsequently. A userinterface 58 including an on/off switch 60 allows a patient or clinicianto operate the external charger 50 to start and stop generation of themagnetic field 45. User interface 58 may also include a Light EmittingDiode (LEDs) or other lamps and possibly also a speaker to indicatestatus. A battery 64 provides power for the external charger 50, whichbattery 64 may itself be rechargeable. The external charger 50 can alsoreceive AC power from a wall plug. A hand-holdable housing 66 sized tofit a user's hand contains all of the components.

Power transmission from the external charger 50 to the IMD 10 occurswirelessly and transcutaneously through a patient's tissue 25 viainductive coupling. FIG. 3 shows details of the circuitry used toimplement such functionality. Primary charging coil 52 in the externalcharger 50 is energized via charging circuit 70 with an AC current,Icharge, to create the AC magnetic field 45. This magnetic field 45induces a current in the secondary charging coil 36 within the IMD 10,providing a voltage across coil 36 that is rectified (38) to DC levelsand used to recharge the battery 14, perhaps via a battery charging andprotection circuitry 40 as shown. The frequency of the magnetic field 66can be perhaps 80 kHz or so. When charging the battery 14 in thismanner, is it typical that the housing 66 of the external charger 50touches the patient's tissue 25, perhaps with a charger holding deviceor the patient's clothing intervening, although this is not strictlynecessary.

External charger 50 can also include one or more temperature sensors 72,such as thermistors or thermocouples, which can be used to report thetemperature (Temp) of external charger 50 to its control circuitry 74.The measured temperature can in turn be used to control production ofthe magnetic field 45 such that the temperature remains within safelimits. This is important as the magnetic field 45 can induce Eddycurrent in conductive structures within the external charger 50, whichEddy currents in turn produce heat. Such heat generation carries a riskof aggravating the patient's tissue 25.

Temperature control of the external charger 50 during an IMD chargingsession can occur for example as explained in U.S. Pat. No. 8,321,029,which is described here with respect to FIG. 4. As shown, a maximumtemperature limit, T max, is set at which the external charger 50 willoperate. T max is generally set in accordance with regulations designedto prevent tissue 25 aggravation, and may comprise 41° C. for example.When the external charger 50 is turned on (switch 60), Icharge flowsthrough the primary charging coil 52, which produces the magnetic field45, causing the temperature sensed by the temperature sensor 72 toincrease. When T max is reached, the control circuitry 74 will disablefurther generation of the magnetic field 45 by stopping energizing ofthe primary charging coil 52 (i.e., Icharge=0). This causes the sensedtemperature to decrease. Eventually, the temperature falls to a secondtemperature limit, T min, which may comprise 39° C. for example. At thisminimum temperature, the control circuitry 74 can once again beginenergizing the primary charging coil 52 with Icharge, once againproducing the magnetic field 45 and causing the temperature to increase,etc. In this manner, during latter portions of the charging session whentemperatures are higher, the charging coil 52 and magnetic field 45 areduty cycled on and off, thus keeping the temperature of the externalcharger 50 within a safe range between T max and T min.

In the example of FIG. 3, there is one temperature sensor 72 which isthermally adhered to an inside surface of the external charger 50'shousing 66. In this design, lead wires 77 connect the temperature sensor72 to the electronics (e.g., control circuitry 74) connected to the PCB54, with lead wires 77 passing through a central hole 76 in the PCB 54.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C show different views of an implantable pulse generator, atype of implantable medical device (IMD), in accordance with the priorart.

FIG. 2 shows an external charger being used to charge a battery in anIMD, while FIG. 3 shows circuitry in both, in accordance with the priorart.

FIG. 4 shows how temperature can be controlled in the external chargerof FIG. 2 by duty cycling the generated magnetic field.

FIGS. 5A and 5B show a first example of an improved external chargersystem having an electronics module and charging coil assembly connectedby a cable, in which a thermal diffuser is included within the chargingcoil assembly.

FIGS. 6A and 6B show a top side of the circuit board in the chargingcoil assembly both with and without the thermal diffuser, while FIGS. 6Cand 6D show the bottom side of the circuit board both with and without athermally insulating material.

FIG. 7 shows circuitry within the external charger system.

FIGS. 8A-8C show different examples of how the thermal diffuser may bepositioned within the charging coil assembly.

FIG. 9 shows use of the thermal diffuser in an external charger in whichelectronics and the primary charging coil are integrated within a singlehousing.

DETAILED DESCRIPTION

The inventors see room for improvement in the external charger 50described earlier, particularly as concerns thermal management.

First, because all electrical components and the charging coil 52 areenclosed in a single housing 66, certain conductive structures withinthe housing such the battery 64 and the electronic components 56 are inproximity to the charging coil 52 and the magnetic field 45 itgenerates. Eddy currents will thus be generated in these conductivestructures in response to the magnetic field 45, which generatesadditional heat in the external charger 50. As a result, the externalcharger 50 will more readily approach its maximum temperature set point,T max, as sensed by temperature sensor 72. This limits generation of themagnetic field 45, and ultimately the power that can be delivered to theIMD 10, which limits the speed at which the IMD 10 can be charged.

Second, placement of the temperature sensor 72 on the inside surface ofthe housing 66 is difficult. Lead wires 77 are required to connect thetemperature sensor 72 to the PCB 54, and these lead wires 77 aredelicate and easily damaged or broken.

Third, while placement of the temperature sensor 72 is logical in thatit attempts to sense the temperature of the external charger 50 at aposition that is closest to the patient, the temperature generated bythe external charger 50 is not highest at this position. Thus, theexternal charger 50 may actually experience temperatures that are higherthan what the temperature sensor senses.

To address these concerns, an improved charging system 100 for an IMD 10is shown in FIG. 5A in a plan view and in FIG. 5B in a side and crosssectional view. The charging system 100 is generally similar instructure to the charging system disclosed in U.S. Patent ApplicationPublication 2017/0361113, but includes additional aspects related tothermal management.

Charging system 100 includes two main pieces: an electronics module 104and a charging coil assembly 102 which includes a charging coil 126. Theelectronics module 104 and the charging coil assembly 102 are connectedby a cable 106. The cable 106 may be separable from both the electronicsmodule 104 and the charging coil assembly 102 via a port/connectorarrangement, but as illustrated cable 106 is permanently affixed to thecharging coil assembly 102. The other end of the cable 106 includes aconnector 108 that can attach to and detach from a port 122 of theelectronics module 104, although this end may be permanently affixed aswell.

Electronics module 104 preferably includes within its housing 105 abattery 110 and active circuitry 112 needed for charging systemoperation, some of which are described subsequently with respect to FIG.7. Electronics module 104 may further include a port 114 (e.g., a USBport) to allow its battery 110 to be recharged in conventional fashion,and/or to allow data to be read from or programmed into the electronicsmodule, such as new operating software.

Housing 105 may also carry user interface elements, which as shown inthe side view of FIG. 5B can include an on/off switch 116 to start/stopgeneration of the magnetic field 45 from the primary charging coil 126,and one or more indicators such as LEDs 118 a, 118 b, and 119. In oneexample, LED 118 a is used to indicate the power status of theelectronics module 104, i.e., the status (capacity) of the electronicsmodule 104's battery 110 and whether it needs charging. LED 118 b maysimilarly indicate the status of the battery 14 (FIG. 1C) in the IMD 10being charged, which information may be telemetered from the IMD 10 tothe charging coil assembly 102 in different manners, as explained in theabove-referenced '113 Publication. One or more LEDs 119 may indicate thedegree of coupling/alignment between the charging coil assembly 102 andthe IMD 10 during a charging session, as explained further below. Morecomplicated user interfaces, such as those incorporating a speaker and adisplay, could also be used. User interface elements can be included onother faces of the electronic module's housing 105, and may be placedsuch that they are easily viewed for the therapeutic application at hand(e.g., SCS, DBS).

Electronics are integrated within the housing 105 of the electronicsmodule 104 by a circuit board 120. Active circuitry 112 may includecontrol circuitry 150 for the charging system 100, as shown in FIG. 7.Control circuitry 150 can comprise a microcontroller programmed withfirmware, such as any of the STM32L4 ARM series of microcontrollersprovided by STMicroeletronics, Inc., as described athttp://www.st.com/content/st_com/en/products/microcontrollers/stm32-32-bit-arm-cortex-mcus/stm3214-series.html?querycriteria=productId=SS1580.Control circuitry 150 may also comprise an FPGA, DSP, or other similardigital logic devices, or can comprise analog circuitry at least in partas explained further below. Control circuitry 150 can further comprise amemory programmed with firmware and accessible to a microcontroller orother digital logic device should that logic device not contain suitableon-chip memory.

Charging coil assembly 102 preferably contains only passive electroniccomponents that are stimulated or read by active circuitry 112 withinthe electronics module 104. Such components include the primary chargingcoil 126 already mentioned, which as illustrated comprises a winding ofcopper wire and is energized by charging circuitry 152 (FIG. 7) in theelectronics module 104 to create the magnetic field 45 that providespower to the IMD 10, such as may be used to recharge the IMD 10'sbattery 14. As shown in FIG. 5B, the primary charging coil 126 ismounted to the top side of a circuit board 124 within a housing 125 ofthe charging coil assembly 102. Housing 125 is preferably formed of aplastic material (e.g., polycarbonate) and as shown preferably comprisesa top housing portion 125 a and a bottom housing portion 125 b, whichmay be joined during manufacturing by screwing, snap fitting, ultrasonicwelding, or solvent bonding. Various views of the circuit board 124 withhousing portions 125 a and 125 b removed for easier viewing are shown inFIGS. 6A-6D. One or more bosses 121 (FIG. 5B) may be formed in thebottom housing portion 125 b to secure the circuit board 124 and othercomponents, and thus the circuit board may include one or more holesthat receive the one or more bosses. In FIGS. 6A-6D, a central hole 124a is provided in the circuit board 124 for receiving the central boss121 shown in FIG. 5B. Top housing portion 125 a may also have bosses orother stabilizing components, although this isn't shown. At least someaspects of active circuitry 112 may also appear in the charging coilassembly 102 if desired or convenient. Preferably, the user interface isassociated only with housing 105 of the electronics module, and housing125 of the charging coil assembly has no user interface elementsassociated with it.

Because the housing 125 of the charging coil assembly 102 is relativelythin—with a thickness x of 1.0 cm or less—and because the primarycharging coil 126 is generally located at the center of this thickness,the magnetic field 45 generated by the primary charging coil 126 duringa charging session will generally be the same on both sides of theassembly. Therefore, either the top 125 a or bottom 125 b housingportion may face the patient (and the IMD 10) during a charging session.

Charging coil assembly 102 preferably includes at least one tuningcapacitor 131 mounted to the circuit board 124. Capacitor 131 is coupledto the charging coil 126 (see FIG. 7) to generally tune the resonantfrequency of this L-C circuit (e.g., to 80 kHz). One skilled in the artwill understand that the value of the capacitor 131 (C) connected to thecharging coil 126 will be chosen depending on the inductance (L) of thatcoil, in accordance with the equation f(res)=1/sqrt(2πLC). Tuningcapacitor 131 can be placed in series or in parallel with primarycharging coil 126.

Also present in the charging coil assembly 102 are one or moretemperature sensors, which are labeled 136 a and 136 b depending onwhether such sensors are located on the top or bottom of the circuitboard 124. As shown in FIGS. 5A, 6A and 6C, two temperature sensors 136a are present on the top of circuit board 124, and two temperaturesensors 136 b are present on the bottom of circuit board 124, with eachspaced 90-degrees radially within primary charging coil 126. In otherexamples, temperature sensors may be present only on the top or only onthe bottom of the circuit board 124. For example, only two temperaturesensors 136 a spaced at 180-degrees may be present on the top of thecircuit board 124. The temperature sensors may comprise thermistors, andin one example can comprise TMP112 High-Accuracy, Low-Power, DigitalTemperature Sensors With SMBus and Two-Wire Serial Interface in SOT563,manufactured by Texas Instruments, Inc.

The charging coil assembly 102 may also include one or more sense coils128, as best shown in FIGS. 5B, 6A and 6C. As explained in theabove-referenced '113 Publication, sense coil(s) 128 can be used todetermine the alignment—e.g., the lateral offset—between the primarycharging coil 126 in the charging coil assembly 102 and an IMD 10 beingcharged. During a charging session, some amount of energy in themagnetic field 45 produced by the primary charging coil 126 will coupleto the sense coil(s) 128, and such coupling will be dependent on thepositioning of the primary charging coil 126 relative to the IMD 10.Thus, as the '113 Publication teaches, one or more of a magnitude ofsignal induced in the sense coil(s) 128, a phase angle of the inducedsignal, and a resonant frequency of the charging system as determinedusing the induced signal, can be used to determine the alignment betweenthe primary charging coil 126 and the IMD 10.

During a charging session, if the primary charging coil 126 ismisaligned with the secondary charging coil 36 (FIG. 1B) in the IMD 10,this will reduce the coupling between these coils, and hence reduce theamount of energy the secondary charging coil 36 receives. This will inturn reduce the rate at which the IMD's battery 14 can be charged, ormore generally can render the charging process inefficient. By contrast,when the primary charging coil 126 perfectly overlies the secondarycharging coil 36 (FIG. 1B) in the IMD 10—such as when the central axesof coils 126 and 36 are collinear—the coupling will be high, andcharging will be faster and more efficient. The degree of misalignmentusing measurements taken from the sense coil(s) 128 can be determined bythe control circuitry 150 as explained further below, and indicated atLED(s) 119 to inform the patient whether he should try to move thecharging coil assembly 102 relative to the IMD 10 to better improvecoupling.

Sense coil(s) 128 are preferably within and concentric with the primarycharging coil 126, and are preferably formed as traces within thecircuit board 124. Although sense coil(s) 128 are illustrated ascomprising a single turn, they may comprise a plurality of turns. Sensecoil(s) 128 are preferably wound in a plane parallel to a plane in whichthe primary charging coil 126 is wound.

Cable 106 includes wires 134 that terminate in the charging coilassembly 102 at a circuit board contact portion 133, where the wires maybe soldered onto contact pads or within vias in the circuit board 124.As shown in the circuit diagram of FIG. 7, such wires 134 include wiresto carry differential AC current signals Icharge+ and Icharge− fromcharging circuitry 152 in the electronics module 104 to the primarycharging coil 126 in the charging coil assembly 102 to energize the coilwith an AC current. These wires are preferably Litz wires, which canpass the AC current from the charging circuitry 152 to the primarycharging coil 126 with reduced loss.

Wires 134 also include those that carry temperature data from thetemperature sensors to the electronics module 104. FIG. 7 shows twotemperature sensors 136(1) and 136(2) present in the charging coilassembly 102 which report temperature data Temp1 and Temp2. As notedearlier, different numbers of temperature sensors can be used, and cable106 can include extra wires 134 to accommodate additional temperaturesensors if necessary. Temperature data may also be multiplexed on asingle wire 134. Temperature data Temp1 and Temp2 may comprise either ananalog voltage indicative of the temperature sensed by their respectivetemperature sensors 136(1) and 136(2), or may comprise digital dataindicative of the sensed temperature. This will depend on the nature ofthe temperature sensors and whether they output analog or digital data.If the temperature data is analog, such data is preferably digitizedbefore being presented to a thermal analysis module 156 in theelectronics module 104. Digitization of the temperature data may occurusing an A/D circuit, or A/D inputs of the control circuit 150 may beused if present. As shown, thermal analysis module 156 may comprise aportion of the control circuitry 150 of the electronics module 104,where it operates as firmware programmed into the control circuitry.Thermal analysis module 156 can also comprise analog circuitry in wholeor in part.

Wires 134 within cable 106 may further include wires that carrydifferential voltage signals Va+ and Va− from the ends of the sense coil128 in the charging coil assembly 102, which differential signalsevidence the voltage Va induced on the sense coil and as useful todetermining alignment as discussed earlier. As shown in FIG. 7, signalsVa+ and Va− are reported to a position analysis module 154 present inthe electronics module 104. Position analysis module 154 preferablycomprises part of the control circuitry 150, and may comprise firmwareprogrammed into the control circuitry 150. Signals Va+ and Va− arepreferably digitized before being presented to the position analysismodule 154, with digitization occurring using an A/D circuit, or A/Dinputs of the control circuit 150 if present. As noted above and asdiscussed in detail in the above-referenced '113 Publication, themagnitude of Va, the phase of Va relative to a reference phase (such asthe drive signal provided to the primary coil), and/or the resonantfrequency of the charging system as determined by assessing Va, can beused by the position analysis module 154 to determine charger-to-IMDalignment. The determined alignment may then be indicated to a user viaalignment indicator 119 discussed earlier. If more than one sense coil128 is present in the charging coil assembly 102, additional wires 134may be provided to report the voltage induced across such additionalsense coils (Vb, Vc, etc.) to position analysis module 154.

As noted earlier, charging system 100 includes additional aspects toassist with thermal management. For example, as shown in FIG. 5B, thecharging coil assembly 102 can include a thermal diffuser 123. In oneexample, the thermal diffuser 123 comprises a thermally conductive, softplastic material. For example, the thermal diffuser may comprise anacrylic pad, such as a non-silicone acrylic pad, such as Part No. 5590H,manufactured by 3M, Inc. See http://multimedia.3m.com/mws/media/920112O/3mtm-thermally-conductive-acrylic-interface-pad-5590h.pdf.Thermal diffuser 123 preferably has a tacky surface allowing it to bepressed onto and adhered to the top of the circuit board 124 and anycomponents on this top surface, including temperatures sensors 136 a.Thermal diffuser 123 preferably has a high thermal conductivity ofgreater than 1.0 W/m-K, and more preferably about 3.0 W/m-K. Thermaldiffuser 123 also preferably has low electrical conductivity, and mayhave a dielectric constant of about 5-6 in one example. Thermal diffuser123 preferably has a thickness t greater than or equal to a thickness ofthe primary charging coil 126, which thickness t may range from 0.5 to4.0 mm for example.

There is not a substantial amount of conductive structures proximate tothe primary charging coil 126 in the design of the charging coilassembly 102, and thus it is not as subject to the generation of Eddycurrents and heat as occurred in the prior art design discussed in theIntroduction. This means that charging coil assembly 102 is able togenerate higher power magnetic fields without overheating, e.g., withoutreaching a maximum prescribed temperature, T max. Thus, the IMD 10 canbe charged faster and more efficiently by charging system 100.

The structure that generates the most heat in the charging coil assembly102 is the primary charging coil 126 itself, which creates a hotring-shaped area inside of the housing 125. Thermal diffuser 123 acts asa heat sink, and provides a heat transfer path away from the primarycharging coil 126, thus distributing this heat over a larger circulararea. Further, this distributed heat is directed to the temperaturesensors 136 a and/or 136 b, and especially to temperature sensors 136 aon the top of the circuit board which with the thermal diffuser 123 isin direct contact. As such, the temperature sensors are better able toaccurately sense the temperature generated within the charging coilassembly 102. Further, because the temperature sensors 136 a and 136 bare mounted to the circuit board 124 in traditional fashion, lead wires(e.g., 77, FIG. 2) and complicated sensor mounting within the assembly102 is not required as in the prior art design, which simplifiesmanufacturing and increases reliability.

To best distribute the primary charging coil 126's heat, it is preferredthat the thermal diffuser 123 be in contact with the primary chargingcoil 126. As best seen in FIGS. 5B and 6B, such contact may occur at anouter edge 123 c of the thermal diffuser 123, which edge 123 c (ofthickness t) is in contact with an inside edge of the primary chargingcoil 126, which has diameter r. Note that FIG. 6A shows the top ofcircuit board 124 without the thermal diffuser 123, while FIG. 6B showsthe top with the material 123 in place.

An advantage to using a soft material for the thermal diffuser 123 isthat it can be cut and shaped. For example, in FIG. 6B a hole 123 a hasbeen cut in the thermal diffuser 123 to accommodate passage of thecentral boss 121 (FIG. 5B). Further, a cut 123 b has been made whichspans from the hole 123 a to the edge 123 c of the thermal diffuser 123.This cut 123 b can accommodate the termination 126 b of the inner end ofthe primary charging coil 126 to the circuit board, which terminationmay not be covered by the thermal diffuser 123. Further, cut 123 ballows the thermal diffuser 123's size to be adjusted slightly to bringthe material into contact with the inner edge of diameter r of theprimary charging coil 126. If due to manufacturing variation this inneredge diameter r is larger than usual, the cut 123 b may be widened toincrease the diameter of the thermal diffuser 123 to allow edge 123 c tocontact the inner edge; if inner edge diameter r is smaller, the cut 123b may be made smaller to decrease the diameter of the thermal diffuser123 to allow edge 123 c to fit within the inner edge.

While it is beneficial to have the thermal diffuser 123 in contact withthe primary charging coil 126 to best assist in distributing its heat,this is not strictly necessary. Heat may conduct from the primarycharging coil 126 to the thermal diffuser 123 even if they are not indirect contact, as the heat may be transferred by intermediaries, suchas the circuit board 124 and the air within the charging coil assembly102. In this regard, although not shown, the thermal diffuser 123 may beconnected to one side of the circuit board 124 while the primarycharging coil 126 is mounted to the other side.

Returning to FIG. 5B, the charging coil assembly 102 may include athermally insulating material 127 on the bottom of the circuit board124. In one example, the thermally insulating material 127 may comprisea foam material, such as a urethane foam, and more particularly maycomprise a Poron™ urethane, manufactured by Rogers Corp. Seehttps://www.rogerscorp.com/ems/poron/index.aspx. Thermally insultingmaterial 127 serves different purposes. First, it can have tackysurfaces on both of its sides, which can adhere to the bottom side ofthe circuit board 124 and to the inside surface of the bottom housingportion 125 b. This helps to stabilize the components within thecharging coil assembly 102, and dampens mechanical shock to protect thecharging coil assembly 102 from damage (e.g., if dropped).

Second, the thermally insulating material 127 will prevent heat from theprimary coil 126 and distributed by the thermal diffuser 123 fromreaching the inside surface of the bottom housing portion 125 b. If thebottom housing portion 125 b faces the patient during a chargingsession, the thermally insulating material 127 will prevent heat fromreaching the patient, which is preferred for safety reason. Preferably,the thermal conductivity of thermally insulating material 127 is 0.2W/m-K or less. Note that FIG. 6C shows the bottom of circuit board 124without the thermally insulating material 127, while FIG. 6D shows thebottom with the material 127 in place. Thermally insulating material 127may be thinner than the thickness t of the thermal diffuser, and may be1.5 mm or less.

If the charging coil assembly 102 b includes temperature sensors 136 bon the bottom side of the circuit board 124 as illustrated, holes 127 b(FIG. 6D) can be provided in the thermally insulating material 127through which the temperature sensors 136 b emerge. This can help todistribute heat to these sensors 136 b. Further, holes 127 b allow thetemperature sensors 136 b to be brought into physical and thermalcontact with the inner surface of bottom housing portion 125 b. This canbe useful, because—similarly to the prior art design of FIG. 2—thesetemperature sensors 136 b will sense a temperature that is closest tothe patient (assuming that portion 125 b contacts the patient during acharging session). Having said this, use of sensors 136 b on the bottomof the circuit board 124 is not strictly necessary, and thus holes 127 bmay not be necessary.

Much of the circuitry of the charging system 100 depicted in FIG. 7 hasalready been discussed, but some extra comments are made here as regardsthermal management and control of the system. As discussed earlier,temperature data (e.g., Temp1, Temp2, etc.) is preferably provided fromthe one of more temperature sensor 136 to the thermal analysis module156 in the electronics module 104. If more than one temperature sensoris present, the thermal analysis module 156 can determine a singletemperature indicative of the temperature of the charging coil assembly.Such single temperature can be for example the average of the reportedtemperature data from each sensor, or (more conservatively from a safetystandpoint) the highest of the reported temperature data.

The single temperature may then be compared to T max and T min as storedin memory associated with the thermal analysis module 156 to controlgeneration of the magnetic field 45. Such control may occur as describedearlier with reference to FIG. 4, with the control circuitry 150instructing the charging circuitry 152 to stop energizing the primarycharging coil 126 when T max is reached (allowing the temperature todecrease), and instructing the charging circuitry 152 to again beginenergizing the primary charging coil 126 when T min is reached (allowingthe temperature to increase). Thus, as before, the charging coil 126 maybe duty cycled on and off, thus keeping the temperature of the externalcharger 50 within a safe range between T max and T min. This is howeverjust one example of control, and thermal analysis module 156 and controlcircuitry 150 more generally may control charging circuitry 152 toenergize the primary charging coil 126 in different manners depending onthe reported temperature.

Thermal diffuser 123 can be differently positioned in the charging coilassembly 102. For example, in FIG. 8A, the thermal diffuser 123 overliesand is in contact with the top of the primary charging coil 126. Thisconfiguration also helps to distribute heat from the primary chargingcoil 126 to the thermal diffuser 123. The thermal diffuser 123 may alsobe pressed to be in contact with the inner edge of the primary chargingcoil 126 as discussed earlier, and may also contact the outer edge ofthe primary charging coil, although this isn't illustrated. Note thatthe thickness x of the charging coil assembly 102 may need to beincreased in this example to accommodate the additional thickness t ofthe thermal diffuser 123 on top of the primary charging coil 126.

In FIG. 8B, a second thermal diffuser 123′ has been added to the bottomof the circuit board 124. This second thermal diffuser 123′ may take theplace of the thermally insulating material 127 as shown in FIG. 8B, ormay occur between the circuit board 124 and the thermally insulatingmaterial 127 (not shown). Use of a second thermal diffuser 123′ assistsin further distributing heat away from the primary charging coil 126 andtowards the temperature sensors, in particular sensors 136 b residing onthe bottom of the circuit board 124. If a thermally insulating material127 isn't used, consideration may need to given to whether the secondthermal diffuser 123′ should be in contact with the inside surface ofthe bottom housing portion 125 b; if in contact with the inside surface,heat will also be transmitted to the bottom housing portion 125 b andalso to the patient, which may or may not be tolerable. Thermaldiffusers 123 and 123′ may have the same thickness, or differentthicknesses (t1, t2) as shown.

FIG. 8C show a modification involving a split primary charging coilhaving two portions: a portion 126 a on the top of the circuit board124, and a portion 126 b on the bottom of the circuit board. Such asplit coil is explained in detail in U.S. Patent Application Publication2017/0361111. Briefly, the coil portions 126 a and 126 b are connectedin series via the circuit board 124, and so in effect operate similarlyto the single primary charging coil 126 described earlier to generate amagnetic field 45. However, by virtue of the manner in which the coilportions 126 a and 126 b are wound and terminated at the circuit board124, the magnetic field 45 produced has improved radial symmetry anduniformity. In any event, a thermal diffuser may be included with bothcoil portions, and as shown a top thermal diffuser 123 is included withtop coil portion 126 a, and a bottom thermal diffuser 123′ is includedwith bottom coil portion 126 b. Any of the modifications describedearlier may be used with the design of FIG. 8C: for example, one or morethermally insulating materials 127 may be used (e.g., with one on thetop and one on the bottom); the thermal diffusers 123 and 123′ mayoverlie the coil portions 126 a and 126 b; etc.

The charging system 100 employing a thermal diffuser has beenillustrated so far in the context of a two-piece charger system havingan electronics module 104 and charging coil assembly 102 in separatehousings 105 and 125. However, a thermal diffuser may be used in anintegrated external charger design in which all electronics and theprimary charging coil are integrated within a single housing. Forexample, FIG. 9 shows the inclusion of a thermal diffuser 123 in anintegrated external charger 50′ similar to that described earlier in theIntroduction. In this example, the primary charging coil 52 occurs onthe bottom of the circuit board 124 as does the thermal diffuser 123,but either or both could appear on the top side as well. Although notillustrated, any of the modifications described earlier may be used withthe design of FIG. 9.

Although particular embodiments of the present invention have been shownand described, it should be understood that the above discussion is notintended to limit the present invention to these embodiments. It will beobvious to those skilled in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe present invention. Thus, the present invention is intended to coverequivalents that may fall within the spirit and scope of the presentinvention as defined by the claims.

What is claimed is:
 1. An external charger system for an implantablemedical device, the external charger system comprising: a circuit board;a primary charging coil, wherein the primary charging coil, whenenergized, is configured to generate a magnetic field to be received atthe implantable medical device; and a thermal diffuser adhered to thecircuit board, wherein the thermal diffuser is configured to conductheat away from the primary charging coil.
 2. The external charger systemof claim 1, wherein the primary charging coil is mounted to a first sideof the circuit board.
 3. The external charger system of claim 2, whereinthe thermal diffuser is adhered to the first side of the circuit board.4. The external charger system of claim 1, wherein the thermal diffuseris adhered to the circuit board within the primary charging coil.
 5. Theexternal charger system of claim 1, wherein the thermal diffuser isfurther adhered to the primary charging coil.
 6. The external chargersystem of claim 1, wherein the thermal diffuser is further in contactwith at least a portion of the primary charging coil.
 7. The externalcharger system of claim 1, wherein the primary coil has an inner edge,and wherein the thermal diffuser has an outer edge in contact with theinner edge.
 8. The external charger system of claim 1, wherein thethermal diffuser is adhered to a first side of the circuit board, andfurther comprising a thermally insulating material in contact with asecond side of the circuit board.
 9. The external charger system ofclaim 1, further comprising at least one temperature sensor mounted tothe circuit board.
 10. The external charger system of claim 9, whereinthe thermal diffuser is in contact with at least one of the temperaturesensors.
 11. The external charger system of claim 10, wherein the atleast one temperature sensor is mounted to the circuit board, andwherein the thermal diffuser is adhered to the at least one of thetemperature sensors.
 12. The external charger system of claim 1, furthercomprising a first housing, wherein the circuit board, the primarycharging coil, and the thermal diffuser are within the first housing.13. The external charger system of claim 12, further comprising a secondhousing coupled to the first housing by a cable, wherein the secondhousing comprises charging circuitry configured to energize the primarycharging coil via the cable to generate the magnetic field.
 14. Theexternal charger system of claim 13, further comprising at least onetemperature sensor mounted to the circuit board, wherein eachtemperature sensor is configured to provide, via the cable, temperaturedata to control circuitry in the second housing, wherein the controlcircuitry is configured to use the temperature data to controlgeneration of the magnetic field.
 15. The external charger system ofclaim 13, further comprising a user interface associated with the secondhousing, wherein no user interface elements are associated with thefirst housing.
 16. The external charger system of claim 1, wherein thethermal diffuser is adhered to the circuit board using a tacky surfaceof the thermal diffuser.
 17. The external charger system of claim 1,wherein the thermal diffuser comprises a plastic material.
 18. Theexternal charger system of claim 1, wherein the thermal diffusercomprises an acrylic material.
 19. The external charger system of claim1, further comprising at least one sense coil within and concentric withthe primary charging coil, wherein a signal induced in each sense coilis configured to provide information relevant to alignment of theprimary charging coil with the implantable medical device duringgeneration of the magnetic field.
 20. The external charger system ofclaim 19, wherein the at least one sensing soil is formed in the circuitboard.